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A New Cosmological Ruler to track Dark Energy

22 mai 2018

To determine the characterisrtics of our Universe, and in particular the nature of the Dark Energy corresponding to 70% of its content, we must measure the speed of its expansion, and have a well calibrated ruler, to measure the size of the universe at various epochs. A characteristic length scale imprinted in the galaxy two-point correlation function, is the baryon oscillation scale, frozen when the ordinary matter (the baryons) decouple from the photons, about 400 000 yrs after the Big-Bang. But this scale is perturbed with non-linearities in the growth of structures and galaxies. An international team of astronomers, led by researchers from Paris Observatory, has proposed to use another scale, dubbed the "linear point", to serve as a comoving cosmological standard ruler. In contrast to the Baryon Acoustic Oscillation (BAO) peak location, this scale is constant in redshift and is unaffected by non-linear effects to within 0.5 percent precision.

Since the 1970s astrophysicists have sought objects of known size that can allow them to track the expansion history of the Universe. Now a French-American team has uncovered a robust new “cosmological standard ruler” they name the Linear Point embedded in the distances to and between far-off galaxies. By measuring millions of such distances, cosmologists seek to unveil the nature of the so-called Dark Energy, the mysterious fuel driving the accelerated expansion of the universe.

Définition du "point linéaire" (en rouge) dans la fonction de corrélation des galaxies.
Anselmi et al (2018)

The traditional “sound-horizon” ruler long advocated by astrophysicists is known to have changed length slightly over the last 14 billion years or so. This has become problematic as cosmological measurements have become more precise. The conventional solution has been to try to correct for these imperfections using theoretical models. Under the leadership of Stefano Anselmi (Observatoire de Paris & Institut d’Astrophysique de Paris), a team of cosmologists composed of Glenn Starkman (Case Western Reserve University), Pier-Stefano Corasaniti (Observatoire de Paris), Ravi Sheth (University of Pennsylvania) and Idit Zehavi (Case Western Reserve University) proposed a more stable ruler, the Linear Point, that requires no corrections. By applying the Linear Point to existing galaxy catalogues, they have shown that this pristine ruler is a new promising means to unravel the mystery of Dark Energy. These results have been published on Physics Review Letters.

Reference

  • The Linear Point : A cleaner cosmological Standard Ruler, Anselmi, S., Starkman, G. D., Corasaniti, P-S., Sheth, R. K., Zehavi, I. : 2018, Phys. Rev. Letters

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